It comes ahead of a key meeting between Russian foreign minister Sergei Lavrov and US secretary of state John Kerry later on Wednesday (GMT).

At one base in Cape Fiolent, near the city of Sevastopol in southern Crimea, Russian soldiers hold some parts of the base although the missile depot remains in Ukrainian hands, Volodymyr Bova, a defence ministry spokesman in the disputed Black Sea peninsula, told AFP.

Pro-Moscow forces are also in partial control of a second base in Evpatoria, which does not have missiles on its grounds.

Ukrainian soldiers still held the command post and control centre there, said another spokesman for the defence ministry in Kiev, Oleksey Mazepa.

The takeovers seemed to have occurred without any violence, officials said.

Some 20 Russian soldiers, backed by hundreds of pro-Moscow forces, had already tried to occupy the Evpatoria base on Tuesday evening, leading to some skirmishes although no shots were fired.

Russian-speaking Crimea has come under de-facto control by pro-Russian forces since the ousting of pro-Kremlin president Viktor Yanukovych and the installation of a new pro-European government in Kiev.

If they are the self-defence forces created by the inhabitants of Crimea, we have no authority over them.

Sergei Lavrov

Russian president Vladimir Putin, however, continues to deny there are any Russians operating in Crimea, insisting that many of the gunman who have identified as Russian soldiers were in fact "local self-defence forces".

The Russian president was backed by Mr Lavrov, who called for all sides in Crimea to respect the letter of the law, treaties, and the Ukrainian constitution so as to allow calm to be restored.

"If they are the self-defence forces created by the inhabitants of Crimea, we have no authority over them," Mr Lavrov told a news conference in Madrid.

"They do not receive our orders."

Mr Lavrov's meeting with Mr Kerry in Paris will be their first since Ukraine's Moscow-backed president Viktor Yanukovych was ousted after three months of pro-European Union protests which left nearly 100 dead.

The Russian minister accused the United States of issuing sanctions on Ukraine, after Washington revoked the visas of officials allegedly linked to the violence, and the European Union of having "threatened" Ukraine over its decision not to sign a treaty with the bloc.

Mr Lavrov recalled a 1994 US-Russia-British security agreement in which the three sides agreed to respect Ukraine's independence and to refrain from the threat or use of force or economic coercion.

Ukraine boosts security at nuclear power plants

Ukraine is reinforcing security measures at its nuclear power plants in response to Russian threats, Ukraine's ambassador has told the UN atomic agency in a letter seen by AFP.

"Illegal actions of the Russian armed forces on the Ukrainian territory and the threat of force amount to a grave threat to security of Ukraine with its potential consequences for its nuclear power infrastructure," Ihor Prokopchuk said.

"Under these circumstances, the competent authorities of Ukraine make every effort to ensure physical security, including reinforced physical protection of 15 power units in operation at four sites of Ukrainian [nuclear power plants]," Mr Prokopchuk said in the letter to International Atomic Energy Agency chief Yukiya Amano.

He added, however, that the "consequences of the use of military force by the Russian Federation" against the former Soviet state "will be unpredictable".

"Numerous efforts of the Government of Ukraine to establish bilateral contacts with the Russian counterparts have not been responded to so far," he said.

He urged Mr Amano to "urgently raise the issue of nuclear security with the authorities of the Russian Federation".